linux-kernel - Re: + espfix-code-cleanup.patch added to -mm tree

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Date:	Wed, 02 Aug 2006 11:30:08 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Stas Sergeev <stsp@...et.ru>
Cc:	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: + espfix-code-cleanup.patch added to -mm tree

Stas Sergeev wrote:
> Hi,
>
> Zachary Amsden wrote:
>> You need to get a #GP or #NP on the faulting iret.  Several ways to 
>> do that -
> I do that much simpler - I provoke a SIGSEGV and in a signal handler
> I put the wrong value to scp->cs or scp->ss, and that makes iret to 
> fault.

Ok, that's a new trick ;)

>
>> iret faults, but doesn't pop the user return frame.
> But does it push the kernel frame after it or not?
> If not - I don't understand how we go to a fixup.
> If yes - I don't understand how the user's frame gets
> accessed later, as it is above the kernel's frame.

Yes.  The iret faults, the fault pushes a new kernel frame - and the 
fault handler's iret returns, removing the kernel frame.  So the kernel 
frame is gone by the time the fixup runs.

>
>>> safe limit is regs->esp + THREAD_SIZE*2... Well, may just I not do 
>>> that please? :)
>>> For what, btw? There are no such a things for __KERNEL_DS or 
>>> anything, so
>>> I just don't see the necessity.
>> It helps track down any bugs that could leak through otherwise and 
>> corrupt random memory.
> I think regs->esp + THREAD_SIZE*2 is already very permissive,
> and I'd like to avoid messing with granularity. So unless you
> really insist, I'll better not do that. :)

It's really hard to catch bugs that could otherwise happen when a 
non-zero based stack gets used (for example, C code which uses %ebp with 
-fomit-frame-pointer).  Setting the limit to THREAD_SIZE should 
guarantee that the non-zero based stack never is used to access anything 
but the stack and current thread.

Zach

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